I am another person who agrees that the 40% for the mount is much too little. The carpet is that fast and you don't need to pay 10p.
Suggestions I have seen I like- Make it so once you finish the quest, the merchant can see what level guild you're in. If you're in a level 50 you can pick a mount higher than 40%, same for a lvl 60 guild.
Or make it so if you buy a horse, you can trade it in for a warg with the same speed as the horse.
Or after you get the mount, add an adornment that can make it faster or a long quest like claymore or something.
Thanks for reading 